This invention relates to an electrochemical battery cell with an aqueous alkaline electrolyte and a negative electrode casing in contact with the negative electrode.
Aqueous alkaline cells are used in many batteries. In the past mercury was often used in these cells to minimize the generation of hydrogen gas due to undesired electrochemical reactions within the cells. Due to environmental concerns, efforts have been under way to reduce or eliminate the addition of mercury to alkaline cells. As a result of other changes in materials and cell designs, most large consumer alkaline batteries currently on the market contain no added mercury. However, the elimination of mercury has been more of a challenge in some types of aqueous alkaline cells, particularly button cells and metal-air cells of all sizes.
Causes of leakage in aqueous alkaline cells include hydrogen gas evolution, wicking of electrolyte through seal members and sealing interfaces and electrochemically driven creepage of electrolyte through sealing interfaces. Hydrogen gas, which can be generated by the corrosion of the negative electrode active material and other metals (including current collectors and contaminants) in contact with alkaline electrolyte, can result in increased internal cell pressure, which can drive electrolyte through weak areas in the cell housing. Electrolyte can wick by capillary action through pores and other openings in seal members and between seal members and other components that seal electrolyte within the cell housing. Capillary wicking can be accelerated by electrocapillary drive when a metal at a sealing interface is at a electrical potential (e.g., a metal component of the cell container in electrical contact with one of the electrodes), since a charged surface is more easily wetted by electrolyte. Electrochemically driven creepage can occur between a seal member and a metal housing component at a negative potential (e.g., a negative electrode casing, cover or current collector). This creepage is the result of hydroxyl ion production from the reduction of oxygen and/or the reduction of water on the negatively charged metal substrate at the leading edge of the electrolyte. The negative potential of the metal substrate, in combination with the localized high concentration of hydroxyl ions can draw water and/or electrolyte into this reaction zone, increasing the volume of liquid and forcing the liquid layer farther from the bulk electrolyte in the cell. This phenomenon is described in detail by Hull et al., in “Why Alkaline Cells Leak”, J. Electrochem. Soc.: Electrochemical Science and Technology, vol. 124, no. 3 (March 1977), p. 332-339; by Davis et al. in “Aspects of Alkaline Cell Leakage”, J. Electrochem. Soc.: Electrochemical Science and Technology, vol. 125, no. 12 (December 1978), p. 1919-1923; and by Baugh et al. in “A Mechanism for Alkaline Cell Leakage”, Journal of Applied Electrochemistry, 8 (1978), p. 253-263.
Mercury worked well to inhibit hydrogen gassing and electrochemically driven creepage because it has a very high hydrogen evolution overpotential, and it may enhance the quality of the plating of negative electrode active material (e.g., zinc) onto the negative electrode current collector, which is important for long term suppression of gassing of the collector. Reduction and elimination of mercury from alkaline cells requires other approaches to minimizing hydrogen gassing. Examples of general approaches that have been used for alkaline Zn/MnO2 cells are disclosed in the following publications, all of which are hereby incorporated by reference: U.S. Pat. No. 5,464,709 (low-gassing zinc compositions); U.S. Pat. Nos. 4,791,036 and 4,992,343 (current collector alloy composition); U.S. Pat. Nos. 5,188,869, 5,281,497, 5,397,658 and 5,445,908 (coating the current collector with low-gassing metals); U.S. Pat. No. 5,168,018 (inorganic gassing inhibitor and organic surfactant added to the negative electrode); and U.S. Pat. No. 6,468,691 (improved seal design) and U.S. Pat. No. 6,256,853 (improved cell closing process).
Because button cells and metal-air cells generally have greater potential for electrolyte leakage and/or the impact of leakage from the cells is greater than in other alkaline cells, elimination of mercury from button and metal-air cells is more of a challenge. Button cells are often sold as single cell batteries with no jackets covering the external surfaces of the cells, leaving portions of the seal interfaces, so any liquid electrolyte or electrolyte salt reaching the outside of seal interfaces is present on the external surface of the battery. Even minute amounts of electrolyte liquid or salt on the external surface of button cells is objectionable from both an aesthetic and a functional standpoint, since it can lead to corrosion of electrical contact surfaces. Metal-air cells and batteries have air access ports in their casings to allow air to enter the cells. These openings are additional places through which electrolyte can leak, and metal-air cells do not always have external jackets to conceal and contain leaking electrolyte. In button cells and some metal-air cells the negative electrode casings can function as current collectors, and the casings can have relatively large surface areas for contacting the negative electrodes relative to the electrode volumes. These large surface areas can provide more opportunity for gassing within the cells.
Clad metals, such as triclad nickel/stainless steel/copper (Ni/ss/Cu), have been popular materials for negative electrode casings for button metal-air cells. The stainless steel core layer provides strength, and the nickel plated outer layer provides an attractive appearance. The copper inner layer has excellent electrical conductivity, provides a continuous coating over the stainless steel, can be readily plated with zinc when contacted by a negative electrode containing an alkaline electrolyte and zinc as an active material, and can be formed into the desired shape without cracking to expose the stainless steel layer beneath.
Attempts have been made to further improve the leakage resistance of alkaline button and metal-air cells in which the negative electrode casing serves as the current collector. For example, Mansfield et al. (U.S. Pat. Nos. 5,279,905 and 5,306,580, which are hereby incorporated by reference) disclose an alkaline zinc-air button cell with a negative electrode casing (anode cup) made from a triclad material with a stainless steel base layer, a nickel layer on the external contact terminal surface and a copper layer on the internal negative electrode contact surface. The entire copper surface of the triclad material is coated with indium, bismuth or zinc, which have higher hydrogen generation overvoltages and lower hydrogen gassing rates than copper, but the external surface of the casing is free of indium, bismuth and zinc to maintain an acceptable appearance. However, Guo et al. (U.S. Pat. No. 6,602,629, which is hereby incorporated by reference) disclose that a coating of indium over the copper layer in the sealing interface area of the casing can offset the benefits of an indium coating in the electrode contact area, and that the seal area of the casing should have no non-in situ deposited metal with a hydrogen overvoltage higher than copper in the seal area to improve leakage resistance; the electrode contacting surface of the casing can be coated with such a metal as long the seal area is not. Ramaswami et al. (U.S. Pat. No. 6,830,847, which is hereby incorporated by reference) disclose an alkaline zinc-air cell with a multi-clad negative casing having at least the peripheral edge surface coated with a protective metal coating layer to prevent potential gradients between the dissimilar metals at the edge surface of the casing material and the resultant hydrogen gassing reactions when the edge surface is in contact with the electrolyte. The inside and/or outside surfaces of the casing may be coated with the same protective metal, which is selected from the elemental metals tin, indium, silver, copper or gold or an alloy such as brass, bronze, phosphor bronze, silicon bronze or a tin/lead alloy. Braunger et al. (U.S. Patent Pub. No. 2003/0211387 A1, which is hereby incorporated by reference) discloses an alkaline zinc-air button cell with an anode casing having a steel or stainless steel layer coated on the outside surface with nickel and on the inside surface with copper. To suppress gassing, at least the outer surface is coated with a copper-tin or copper-tin-zinc alloy containing substantially no nickel; the inner surfaces of the negative and positive casings may also be coated with the nickel-free alloy.
There is still a need for further improvement in the leakage resistance of alkaline button and metal-air cells in which the negative electrode casing serves as the current collector, especially cells with no added mercury, without adversely affecting the aesthetic appearance of the cell.
In view of the above, an object of the present invention is to provide an electrochemical battery cell with an aqueous alkaline electrolyte and having a negative casing in contact with the negative electrode with excellent leakage resistance.
Another object of the invention is to provide an alkaline cell, particularly a button or metal-air cell with no added mercury, with excellent resistance to electrolyte leakage resulting from hydrogen gassing at collector surface, electrocapillary leakage and electrochemically driven creepage, while maintaining an excellent aesthetic appearance.